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16 votes
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Are antennas circuits?

What do you mean by "circuit"? Do you mean there's a loop of conductor from one side of a battery to the other, possibly with some other conducting components along the way? How about this circuit? ...
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13 votes
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Why do we use decibels in radio?

We use decibels because they are the most intuitive representation when our primary concern is signal quality. Signal quality changes not in proportion to the power added (in watts), but rather to the ...
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12 votes
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How does signal strength relate to bandwidth?

It depends on what you mean by "stronger". Ignoring actual limitations of your hardware and just considering the theory of communications, if you have 25 W of transmit power, then you can spread that ...
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12 votes

How does IQ modulation work (intuitively)?

I think it's more intuitive if you unlearn some things first. Oscillation is not: $$ \cos(\omega t) $$ where $\omega$ is the angular frequency in radians per second, and $t$ is time. Rather, ...
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10 votes
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Can there be (long range) propagation of HF from satellites?

Satellites can and do use HF for communications. The first example would of course be Sputnik, which transmitted at 20 and 40 MHz. Amateurs use HF to communicate with satellites. According to Amsat, ...
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  • 8,225
8 votes

Why can't VHF / UHF be used with ionosphere reflection?

The ionosphere typically neither reflects nor absorbs waves with VHF or higher frequencies, but passes them through to space. There are no reflections back to the ground, so there is no useful ...
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8 votes
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How does a folded balun work?

It's easier to see what's going on with a bit of rearrangement. Imagine this built of tubing: The feedpoint is still where it would be on a dipole without the balun. The feedpoint sees the dipole as ...
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8 votes
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What is the velocity factor of a wire?

A coaxial cable is a transmission line by itself, and therefore the velocity factor is known. It depends on the dielectric properties, geometry, and conductivity. A single wire is not a transmission ...
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  • 1,099
7 votes
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How does a Faraday cage block an EMP?

Faraday cages block EMP in the same way they block all other time-varying electromagnetic fields. The only difference between blocking EMP versus blocking an ordinary radio transmission is the EMP is ...
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6 votes

What is the velocity factor of a wire?

Velocity factor is a property of electromagnetic wave propagation, not wire. A transmission line (like coax) is a conduit for an electromagnetic wave in itself, so the velocity factor can be defined. ...
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6 votes
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Key CW on zero crossings - zero bandwidth?

You raise an excellent question and your thought processes are indeed on the right track. First some background. An ideal, uninterrupted sinusoidal carrier has zero bandwidth. Real world factors such ...
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6 votes
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Why do we talk about common mode current from an antenna, but not from a transmitter?

The short answer is that it can't. A shielded transmitter, connected to an ideal piece of coax, does not generate common mode currents. The inner and the outer of the cable look connected (and for ...
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6 votes
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What does antenna frequency range mean in practice?

It means the range of frequencies in which the antenna is designed to operate. Operated outside that range, the antenna may not meet its specifications. This particular antenna is a telescopic whip, a ...
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6 votes
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Why do PSK modes have "bandwidth"?

Binary PSK with instantaneous phase shifts would be equivalent to multiplying a sine wave (the carrier) with a square wave with values at 1 or -1. When two signals are multiplied, this forms a ...
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5 votes
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Why must twin-lead conductor spacing be small to avoid radiation?

Twin-lead transmission lines don't radiate because the opposite fields from each conductor cancel, but when the spacing is far apart this does not happen. First, let's consider the magnetic field ...
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5 votes

Can a common-mode current exist on the inside of a coax shield?

It is helpful to understand the basic functioning of a coaxial cable. But first there are two important phenomenons that must be understood in order to proceed. Skin Effect When direct current (time ...
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4 votes
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How is RF a radio receives just not a mishmash of eletromagnetic energy?

Filters. A filter is a circuit which is designed to react differently depending on the frequency. Every real (not mathematically ideal) circuit element does this to some degree, but filters are ...
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4 votes

SDR sampling bandwidth - do the bits per sample matter?

For HF usage with 200 Msps converters and above, an 8 bit ADC will provide very good performance. You write: "the sampling frequency has to be several times the target frequency" but that is not true ...
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  • 899
4 votes

There is no reverse flow of energy in a mismatched transmission line, is this correct?

I'm afraid that it is not correct that there is no energy transfer back to the source after an initial period. Wherever you have a mismatch on a transmission line, there is a reflection (at least ...
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4 votes

What does the term 3 kHz deviation relate to in the FM world?

FM works by varying the frequency of the signal around the nominal carrier frequency. Because the frequency is varying, the signal is not a pure sine wave. Therefore, it necessarily has some energy in ...
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4 votes

Key CW on zero crossings - zero bandwidth?

There is no way to transmit information in a signal with zero bandwidth. Switching the carrier at the zero crossings would reduce bandwidth but not take it to zero. There's a mathematical explanation ...
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4 votes
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Conventional or electron current

Current flow is electron flow and it is from negative to positive. This is what we call class A hogwash. Current notation is just a convention. Going by electron flow is not righter than going the ...
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4 votes
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Does the E-field propagate faster that the H-field?

The E and H fields of all RF signals travel at exactly the same speed - the speed of light. The phase relationship between the two fields remains constant as they travel through various mediums. ...
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4 votes
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Voltage wave in transmission lines

A transmission line is linear, which means we can consider the voltage at any point to be the superposition of two other voltages. Asking what they are "physically" may or may not be a useful ...
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4 votes
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Transmitting Video Underwater

Here is some useful scholarly information about the signal attenuation (in dB) of EM waves through freshwater at various frequencies and depths. You will find a formula that you can use. You will also ...
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  • 7,544
4 votes

How does IQ modulation work (intuitively)?

I believe a good point of view is the concept of orthogonality. This is clear under everybody's eyes when seen in physical space, take for instance a 2-dimension space, a plane. In the example above ...
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  • 173
4 votes

What cause `multiply` has difference result between `float` and `complex` signal?

That's just the math behind it – everything is alright with these results! You need to write down the formula of the real-valued $\sin(t)$ in terms of $e^{j2\pi t}$ and $e^{-j2\pi t}$, and you'll see ...
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3 votes
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What is shape factor of a resonant circuit?

Well, different books / schools use different definitions. Some authors just copy definitions from others and make mistakes when doing that. In this case, this is pretty likely: Possibly, someone ...
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3 votes
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How is impedance matching related to balance and common-mode currents?

The two are separate figures, so one does not imply the other. A good match doesn't imply no common mode currents, and no common-mode current does not imply a good match. They are somewhat connected -...
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3 votes

Are antennas circuits?

The idea that a closed circuit — a loop — must be present for current to flow is a simplified description, which is only true for simple DC circuits. It's a simplification that works because in the DC ...
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